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I struggled to find out much on the National Archives catalogue.
Ancestry has the Medical Register which will tell you where he qualified and if you work your way back and forth through the years will give his career as a civilian.
He should have a mention in Peterkin, A., and Drew, R, 1968: Commissioned officers in the medical services of the British Army, 1660-1960
You might find out more through War Diaries though he's unlikely to be mentioned by name.
The relevant museum might be able to help: https://www.museumofmilitarymedicine.org.uk/

There are possible lines to follow on PoWs here:
https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/british-prisoners-of-war-1790-1919/

I wondered why probate never mentioned his wife Janet Scott McKay who seems to have died jq 1926,after his 31.3.1926 alleged death, transcript error makes perfect sense.

SS

Even if she were still alive, the probate register only names executors or administrators, who may not be the beneficiaries. Often the executor is a solicitor or a bank; or someone who's prepared to do the work for the family.

The farm we moved to in the 1950s had no electricity in the house. Separate generator for the milking parlour. We installed generators; took a few years for the electricity board to connect us.

see this guide:

https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/doctors-and-nurses/